There are many applications of prime movers to be more or less continuously operating with constant speed or power that is desired to be changed from time to time from one constant speed or power range to another constant speed or power range during the continuous period of operation. For example, transportable refrigeration units are often provided with continuously running internal combustion engines to power their refrigerant compressors and other components during use. As the refrigeration capacity or demand changes it may be desirable to change the constant running speed of the internal combustion engine and therefore a multi-position throttle actuating mechanism of linkage having different stroke lengths that may be quickly and easily selected and changed to change the predetermined constant running speed of the engine is required. For maximum flexibility of control a variable stroke throttle linkage having more than two adjustable pre-set stroke lengths is most desirable so that at least three different pre-set constant operating speeds for the engine may be selectively obtained and predetermined.